[0001] This invention relates to a steel having excellent low temperature toughness at a
welding heat affected zone (HAZ), and can be applied to structural steel materials
to which arc welding, electron beam welding, laser welding, etc, are applied.
[0002] More particularly, this invention relates to a steel having excellent HAZ toughness
by adding Ti and Mg to a steel, controlling an O quantity and finely dispersing oxides
and composite oxides of these elements.
[0003] One of the most important characteristics required for steel materials used for structures
such as ships, buildings, pressure containers, linepipes, etc, is HAZ toughness. Recently,
heat-treating technologies, controlled rolling and machining heat-treating method
have made a remarkable progress, and the improvement of low temperature toughness
of the steel material itself has become easy. Because the welding HAZ is reheated
to a high temperature, however, the fine structure of the steel material is completely
lost, and its microscopic structure becomes extremely coarse to thereby invite drastic
deterioration of HAZ toughness. Therefore, as means for refining the HAZ structure,
(1) a technology for limiting coarsening of austenite grains by TiN and (2) a technology
for forming intergranular ferrite by Ti oxides, have been examined and put into practical
application. For example, CAMP-ISIJ Vol. 3 (1990) 808 describes the influences of
N on intergranular ferrite transformation in Ti oxide type steels, and Vol. 79 (1993)
No. 10 describes the effect of B on intergranular ferrite transformation in Ti-containing
oxide steels. JP-A-7-216496 and JP-A-7-216498 disclose a fire-resistant and ductile
rolled section containing 0.001-0.010% Mg and composite oxides of Al-Mg-Ti system
having a size of not greater than 3µm. JP-A-5-43977 discloses a method for producing
a steel having excellent HAZ toughness by uniformly dispersing titanium oxides in
a great quantity as nuclei of interaranular ferrite grains and the steel contains
0.005-0.020% Ti and 0.001 - 0.010% Mg. Further, EP-A-0 757 113, which corresponds
to WO-A-96/23909, discloses a high strength and low-yieid-ratio line pipe steel having
excellent HAZ toughness containing 0.005-0.030% Ti, 0.001-0.006% Mg and having a microstructure
with a specific ferrite fraction and a mean grain size. Nonetheless, the level of
HAZ toughness produced by these technologies is not yet entirely satisfactory. From
the aspect of execution of welding, therefore, a steel material which has higher strength
and can be used at a low temperature and with a large heat input has been strongly
desired.
[0004] The present invention provides a steel material having excellent HAZ toughness (such
as thick steel plates, a hot coil, a shape steel, a steel pipe, etc).
[0005] The inventors of the present invention have conducted intensive studies on the chemical
components (compositions) of steel materials and their microscopic structures in order
to improve their HAZ toughness, and have invented a novel steel having high HAZ toughness.
[0006] The object of the present invention is solved by the features specified in the claims.
[0007] Hereinafter, the content of the present invention will be explained.
[0008] The term "%" used in the following description means "wt%".
[0009] The feature of the present invention resides in that trace Ti and Mg are simultaneously
added to a low carbon steel and oxides and composite oxides containing Ti and Mg (containing
additionally MnS, CuS, TiN, etc) are finely dispersed into the steel by controlling
the O (oxygen) quantity.
[0010] Here, the term "oxides and composite oxides containing Ti and Mg (containing additionally
MnS, CuS, TiN, etc)" mainly means compounds such as Ti oxides, Mg oxides or composite
oxides of Ti and Mg in the steel, oxides and composite oxides of other elements such
as Mn, Si, Al, Zr, etc, and compounds such as sulfides and composite sulfides of Mn,
Cu, Ca, Mg, etc. These compounds may further contain nitrides such as TiN.
[0011] It has been clarified that the finely dispersed Ti and Mg-composite oxides restrict
(1) formation of fine intergranular ferrite in the austenite grains that have become
coarse and/or (2) coarsening of the austenite grains, make the HAZ structure fine
and drastically improve the HAZ toughness. Moreover, the improvement of the HAZ toughness
can be attained by the Mg quantity in the steel and the kind of the Mg addition elements.
In other words, it has been found out that when pure Mg metal (at least 99%) is wrapped
by an iron foil and is added, the item (1) exhibits its effect when the Mg quantity
is not greater than 0.0020% and the item (2) exhibits its effect when the Mg quantity
exceeds 0.0020%. In addition, the sizes and densities of the Ti and Mg composite oxides
are important factors.
[0012] However, there is the case where the oxide of Mg alone exists besides the Ti and
Mg composite oxide when the Mg quantity is great, and there is also the case where
the oxide of Ti alone exists besides the Ti and Mg composite oxide when the Mg quantity
is small. However, there occurs no problem so long as the sizes of the individual
oxides of Ti and Mg and the Ti and Mg composite oxides are from 0.001 to 5.0 µm because
they are finely dispersed. The sizes of the oxide or the composite oxide are preferably
0.001 to 2 µm.
[0013] It has been also clarified that this composite oxide is dispersed in a greater quantity
and more finely than the Ti oxide formed at the time of addition of Ti alone, and
its effects on the items (1) and (2) described above are also greater. To obtain such
effects, it is first necessary to limit the Ti and Mg quantities to 0.005 to 0.025%
and 0.0001 to 0.0009, respectively. These quantities are the minimum quantities necessary
for finely dispersing large quantities of the composite oxides. The upper limit of
the Ti quantity must be 0.025% in order to prevent deterioration of the low temperature
toughness due to the formation of TiC at the HAZ, though the Ti quantity varies with
the O and N quantities. It is extremely difficult from the aspect of steel production
to disperse large quantity of Mg oxides and for this reason, the upper limit of the
Mg quantity is set to 0.0009.
[0014] When the size of the Ti and Mg composite oxide is less than 0.001 µm, the oxide is
so small that the restriction effect of coarsening of the austenite grain or the formation
effect of the intergranular ferrite cannot be obtained. When the size exceeds 5.0
µm, the oxide is so large that the restriction effect of coarsening of the austenite
grains or the formation effect of the intergranular ferrite cannot be obtained, either.
When the density of the Ti and Mg composite oxide is less than 40 particles/mm
2, the number of oxides dispersed is so small that the effect of intergranular transformation
cannot be obtained. Therefore, the density of at least 40 particles/mm
2 is necessary. To obtain finer Ti and Mg oxides in greater quantities, limitation
of the O quantity is important. When the O quantity is too small, large quantities
of the composite oxides cannot be obtained and when it is too great, on the contrary,
the cleanness of the steel is deteriorated. Therefore, the O quantity is limited to
0.001 to 0.004%.
[0015] Hereinafter, the reasons for limitation of the component elements will be explained.
[0016] The C quantity is limited to 0.01 to 0.15%. Carbon is an extremely effective element
for improving the strength of the steel, and at least 0.01% is necessary so as to
obtain the fining effect of the crystal grains. When the C quantity is too great,
the base metal and the low temperature toughness of the base metal and the HAZ are
extremely deteriorated. Therefore, the upper limit is set to 0.15%.
[0017] Silicon is the element added for deoxidation and for improving the strength. When
its quantity is too great, however, the HAZ toughness is remarkably deteriorated,
and the upper limit is therefore set to 0.6%. Deoxidation of the steel can be made
sufficiently even by Ti or Al, and Si need not be always added.
[0018] Manganese is an indispensable element for securing the balance of strength and the
low temperature toughness and its lower limit is 0.5%. When the Mn quantity is too
great, however, hardenability of the steel increases, so that not only the HAZ toughness
is deteriorated but center segregation of continuous casting (slab) is promoted and
the low temperature toughness of the base metal is deteriorated, too. Therefore, the
upper limit is set to 2.5%.
[0019] The addition of Ti forms fine TiN, restricts coarsening of the austenite grains at
the time of reheating of the slab and the HAZ, makes fine the microscopic structure
and improves the low temperature toughness of the base metal and the HAZ. When the
Al quantity is small, Ti forms oxides, functions as the intergranular ferrite formation
nuclei in the HAZ and makes fine the HAZ structure. To obtain such a Ti addition effect,
at least 0.005% of Ti must be added. If the Ti quantity is too great, however, coarsening
of TiN and precipitation hardening due to TiC occur. Therefore, its upper limit is
set to 0.025%.
[0020] Aluminum is the element which is generally contained in the steel as the deoxidizing
element. However, when the Al quantity exceeds 0.02%, the Ti and Mg composite oxides
cannot be easily formed. Therefore, its upper limit is set to 0.020%. Deoxidation
can be sufficiently achieved by Ti or Si, and Al need not always be added.
[0021] Magnesium is a strong deoxidation element and forms fine oxides (composite oxides
containing trace Ti, etc) when it combines with oxygen. The Mg oxides finely dispersed
in the steel are stabler even at a high temperature than TiN, restrict coarsening
of the gamma-grains in the entire HAZ or form the fine intergranular ferrite inside
the coarsened austenite grains, and improve the HAZ toughness. To obtain such effects,
at least 0.0001% of Mg is necessary. However, it is extremely difficult from the aspect
of steel production to add a large quantity of Mg into the steel. Therefore, its upper
limit is set to 0.0009.
[0022] It is effective to reduce the quantity of the strong deoxidation element Al as much
as possible and to control the O quantity to 0.001 to 0.004% in order to sufficiently
obtain the fine oxides at the time of the addition of Ti and Mg.
[0023] Nitrogen forms TiN, restricts coarsening of the austenite grains at the time of reheating
of the slab and in the welding HAZ, and improves the low temperature toughness of
the base metal and the HAZ. The minimum quantity necessary for this purpose is 0.001%.
When the N quantity is too great, however, surface scratching of the slab and deterioration
of the HAZ toughness due to solid solution N occur. Therefore, the upper limit must
be set to 0.006%.
[0024] In the present invention, the P and S quantities as the impurity elements are limited
to not greater than 0.030% and not greater than 0.005%, respectively. The main reason
is to further improve the low temperature toughness of the base metal and the HAZ.
Reduction of the P quantity reduces the center segregation of the slab, prevents the
grain boundary destruction and improves the low temperature toughness. Reduction of
the S quantity reduces MnS stretched by controlled rolling and improves the toughness.
[0025] Next, the object of the addition of Nb, V, Ni, Cu, Cr and Mo will be explained.
[0026] The main object of addition of these elements to the fundamental components is to
further improve the characteristics such as the strength/low temperature toughness,
HAZ toughness, etc, and to enlarge the producible steel size without deteriorating
the excellent features of the steel of the present invention. Therefore, their addition
quantities must be naturally limited.
[0027] When co-present with Mo, Nb restricts recrystallization of the austenite during controlled
rolling, makes fine the crystal grains but contributes to the improvement of precipitation
hardening and hardenability and makes the steel tough and strong. At least 0.005%
of Nb is necessary. When the Nb addition quantity is too great, however, the HAZ toughness
is adversely affected. Therefore, its upper limit is set to 0.10%.
[0028] Vanadium has substantially the same effect as Nb but its effect is believed to be
weaker than that of Nb. At least 0.01% of V must be added, and the upper limit is
set to 0.10% from the aspect of the HAZ toughness.
[0029] Nickel is added in order to improve the strength and the low temperature toughness.
It has been discovered that in comparison with the addition of Mn, Cr and Mo, the
addition of Ni forms less of the hardened structure, which is detrimental to the low
temperature toughness, in the rolled structure (particularly, in the center segregation
zone of the slab) and the addition of a trace quantity of Ni is also effective for
improving the HAZ toughness (a particularly effective Ni quantity for the HAZ toughness
is at least 0.3%.) If the addition quantity is too great, however, not only the HAZ
toughness is deteriorated but the economic effect is also spoiled. Therefore, its
upper limit is set to 2.0%. The addition of Ni is also effective for preventing Cu
cracking during continuous casting and hot rolling. In this case, Ni must be added
in a quantity of at least 1/3 of the Cu quantity.
[0030] Copper has substantially the same effect as Ni and is effective for improving corrosion
resistance and hydrogen induced cracking resistance characteristics. The addition
of Cu in a quantity of at least about 0.05% drastically improves the strength due
to precipitation hardening. When it is added excessively, however, a drop in the toughness
of the base metal and the HAZ due to precipitation hardening and the occurrence of
crack during hot rolling develop due to precipitation hardening. Therefore, its upper
limit is set to 1.2%.
[0031] Chromium increases the strength of the base metal and the welded portion. However,
when its quantity is too great, the HAZ toughness is remarkably deteriorated. Therefore,
the upper limit of the Cr quantity is set to 1.0%.
[0032] Molybdenum strongly restricts recrystallization of the austenite during controlled
rolling when co-present with Nb, and is effective also for fining the austenite structure.
However, the excessive addition of Mo deteriorates the HAZ toughness, and its upper
limit is set to 0.8%.
[0033] The lower limit of 0.05% of each of Ni, Cu, Cr and Mo is the minimum quantity at
which the effect on the material due to the addition of these elements becomes remarkable.
[0034] Next, the size and the number of the Ti and Mg composite oxide particles will be
explained.
[0035] When the size of the Ti and Mg composite oxide particles is less than 0.001 µm, the
effect of the formation of the intergranular ferrite or the restriction effect of
coarsening of the austenite grains cannot be obtained, and when it exceeds 5.0 µm,
the oxide particles become so large that the oxide does not provide the formation
effect of the intergranular ferrite, and the restriction effect of coarsening of the
austenite grains cannot be obtained.
[0036] When the density of the Ti and Mg composite oxide particles is less than 40 particles/mm
2, the number of the oxide particles dispersed is small and the oxide particles are
not effective for intergranular transformation. Therefore, the lower limit is set
to at least 40 particles/mm
2.
[0037] By the way, the density of the oxides of Ti and Mg alone or their composite oxide
is determined by collecting a sample from a position of 1/4 thickness, irradiating
a beam of a 1 µm diameter to the range of 0.5 mm x 0.5 mm on the sample surface by
using a CMA (Computer Micro-Analyzer) and calculating the number of oxide particles
per unit area.
[0038] Next, the Mg addition material will be explained. The present invention uses metallic
Mg (at least 99%) wrapped by an iron foil as the Mg addition material and melts it
to a steel. If metallic Mg is directly charged into the molten steel, the reaction
is so vigorous that the molten steel is likely to scatter. Therefore, metallic Mg
is wrapped in the iron foil. The reason why the iron foil is used is to prevent impurity
elements from entering the molten steel but no problem occurs when the foil of an
iron alloy having substantially the same composition as that of the product is used.
Incidentally, a Mg alloy such as an Fe-Si-Mg alloy or a Ni-Mg alloy may be used as
the Mg addition material.
Best Mode for Carrying Out the Invention:
[0039] Ingots of various Mg-containing steels, to which pure Mg metal (at least 99%) was
added while being wrapped by an iron foil, were produced by laboratory melting. These
ingots were rolled to plates having thickness of 13 to 30 mm under various conditions
and their mechanical properties were examined. The mechanical properties (yield strength:
YS, tensile strength: TS, absorption energy of Charpy impact energy at -40°C: vE
-40 and Charpy impact transition temperature: vTrs) were examined in a transverse direction.
The HAZ toughness (Charpy impact energy at -20°C: vE
-20) was evaluated by HAZ reproduced by a reproduction heat cycle apparatus (maximum
heating temperature: 1,400°C, cooling time from 800 to 500°C [Δt
800-500]: 27 sec). The sizes and numbers of the Ti and Mg composite oxide particles were
examined by effecting CMA analysis using a 1 µm diameter beam.
[0040] The oxide particles were determined by electron microscope observation.
[0041] Examples were tabulated in Table 1. The steel sheets produced in accordance with
the present invention had Charpy impact energy of at least 150 J in the HAZ at -20°C,
and had excellent HAZ toughness. In contrast, since the Comparative Steels had unsuitable
chemical components or unsuitable sizes or densities of the Ti and Mg composite oxide
particles, their Charpy impact energy in the HAZ at -20°C was extremely inferior.
[0043] The present invention can stably mass-produce a steel material which has excellent
HAZ toughness and can be used for structures such as ships, buildings, pressure containers,
linepipes, and so forth. As a result, the safety of ships, buildings, pressure containers
and pipelines can be remarkably improved.